The tracking and location of assets such as railcars, shipping or cargo containers, trucks, truck trailers, automobiles, etc., can be highly advantageous in commerce. Precise tracking of such vehicles and objects can facilitate their being efficiently allocated and positioned, and can provide for immediate, accurate localization of lost, delayed or damaged assets. The space-based global positioning system (GPS), implemented by the United States Department of Defense, constitutes a convenient instrumentality for determining geographical position in real time.
The GPS is a multiple-satellite-based radio-positioning system. Each GPS satellite transmits data to be used by a receiver to measure its distance from the GPS satellite. The signals from several satellites allow the receiver to compute its position, velocity and time parameters through known triangulation techniques. The signals provided by the GPS can be received globally and continuously.
The space segment consists of twenty-one operational satellites and three spare satellites. The satellites are positioned in orbits such that typically seven satellites, but a minimum of four, are observable at any time by a user anywhere on or near the earth's surface if the user has a clear view from the zenith to down near the horizon. Each satellite transmits signals on two frequencies known as L1 (1575.42 MHZ) and L2 (1227.6 MHZ), using direct-sequence spread spectrum techniques. A system with two tiers of position accuracy is provided by employing both C/A (coarse or clear/acquisition) and P (precise) spreading codes. Both C/A and P pseudo-random noise (PRN) codes are transmitted on frequency L1, and only the P code is transmitted on frequency L2. The C/A is available to any user, military or civilian, but the P code is usually encrypted (becoming Y code) and usable only by authorized military and civilian users. Both codes contain the timing information needed for determining the range from a satellite to the Superimposed on both the P and C/A codes is a 50 bit/second navigation (NAV) data stream that is unique for each satellite. Each NAV data stream is a continuous sequence of 30-second message frames, organized as a sequence of five 6-second sub-frames. Each sub-frame begins with a synchronization sequence called the telemetry-word preamble (TWP). This is followed by a hand-over word (HOW) that indicates GPS time at the beginning of the next sub-frame, and facilitates the transition from C/A to P code tracking. Each frame includes accurate ephemeris data that describes the satellite's position as a function of time, and clock-correction data for that satellite. Spread over twenty-five adjacent frames is the GPS almanac data for the entire set of GPS satellites. The almanac data includes approximate ephemeris data, satellite health status, coefficients for the ionospheric delay model for C/A code users, and coefficients used to calculate universal coordinated time (UCT).
The control segment comprises a master control station (MCS) and a number of monitor stations. The monitor stations track all GPS satellites, collecting ranging data and satellite clock data from each satellite. This information is passed on to the MCS where the satellite's future ephemeris and clock drift are predicted and, sometimes, corrected. The purpose of the control segment is to ensure that the information transmitted from the satellites is as accurate as possible.
The GPS is intended to be used in a wide variety of applications including space, air, sea and land vehicle navigation, precise positioning, time transfer, altitude referencing and surveying. A typical GPS receiver comprises a number of subsystems, including an antenna assembly, an RF (radio frequency) assembly, and a GPS processor assembly. The antenna assembly receives the L-band GPS signal and amplifies it prior to insertion into the RF assembly. A significant factor affecting accuracy of the computed position, velocity or time parameters is the relative positional geometry of the satellites selected for measurement of ranges. Generally, a best position solution is obtained using satellites having wide angles of separation. Considerable emphasis has therefore been placed on designing antenna systems to receive, with uniform gain, signals from any point on the hemisphere.
In a typical GPS receiver, the RF assembly mixes the L-band GPS signal down to a convenient IF (intermediate frequency) signal. Using various known techniques, the PRN code modulating the L-band signal is tracked through code-correlation at the receiver. This provides the processing gain needed to achieve a signal-to-noise ratio (SNR) sufficient for demodulating the navigation data. The Doppler shift of the received L-band signal is also measured through a carrier tracking loop. The code correlation and carrier tracking function can be performed using either analog or digital signal processing.
A typical GPS receiver receives and processes signals from several of the GPS satellites in order to determine range to (and relative velocity of) each satellite. With perfect knowledge of range to only three of the GPS satellites, exact receiver position can be determined from the intersection of the three "spheres" induced by the known satellite positions and the derived receiver ranges. With receiver noise and imperfect knowledge of satellite positions, the receiver-satellite ranges can only be estimated. Typically, errors from receiver noise are reduced by (effectively) averaging many range calculations. The pseudo range from a particular satellite is calculated by reading the transmission time (time stamp) associated with the sub-frame time marker in the satellite's data stream, subtracting this time from the reception time of the time marker (as indicated by the receiver time clock), and multiplying the difference by the speed of light. Error in the receiver clock leads to proportional pseudo-range errors. Because the same clock is used in receiving from all satellites, the same clock error is involved in all pseudo-range measurements. By measuring the pseudo-range from four or more satellites, the clock error (bias) and ranges can be jointly estimated.
At the receiver, the reception time of the time marker (or of any NAV data-bit edge) is determined by performing a cross-correlation of the received signal with a local replica of the known satellite PRN code, and noting the receiver time of the cross-correlation peak associated with the marker. The satellite signal structures use Code Division Multiple Access (CDMA) so that the above cross-correlation is part of the standard GPS receiver processing. A description of CDMA techniques may be found, for example, in CDMA Principles of Spread Spectrum Communication, by Andrew J. Viterbi, Addison-Wesley (1995).
A typical GPS receiver must determine its own position. This requires that the GPS time stamp, satellite ephemeris, and other NAV data be decoded from each satellite's data stream at the tracked object. The receiver is thus required to process data from each satellite long enough (between six and 150 seconds) to synchronize with, and decode, these data. This consumes significant energy.
U.S. Pat. No. 5,752,218, issued May 12, 1998 and assigned to the instant assignee, discloses a Reduced Order GPS (ROGPS) system that substantially reduces the energy requirements at tracked objects. This reduction is accomplished by deriving ROGPS data from the received GPS signals at the tracked object and sending these data to the central station for processing. The ROGPS data require less GPS receiver and processor operating time than does conventional GPS data reception; however, the ROGPS data set is larger than that generated by a normal GPS receiver, and hence more data must be communicated to the central station. This increases the communication channel occupancy and transmitter energy consumed.
One reason that the ROGPS data set is larger than that generated by a normal GPS receiver is that the ROGPS data set must include satellite identification data indicating the GPS satellite indices associated which each ROGPS measurement. It would be advantageous to have an improved ROGPS system that uses fewer data bits to communicate these indices to the central station.